The invention is based on an exhaust system for a multi-cylinder internal combustion engine and, more particularly, to an exhaust system for a multi-cylinder internal combustion engine with a first group of cylinders that cannot be switched off and a second group of cylinders that can be switched off, with exhaust lines associated with each group which start from exhaust manifolds, with at least two catalytic converters in the exhaust system, and with an actuating member in one of the exhaust lines.
German Patent document DE 29 12 796 C2 discloses an exhaust system of the above-mentioned type for a multi-cylinder engine in which one group of cylinders is operated constantly and another group of cylinders can be switched off. Two catalytic converters are disposed in the exhaust system, through which in succession there is a flow of exhaust gas from the constantly operated cylinders. Via an actuating element, the exhaust flow from the cylinder group that can be switched off can be alternatively conducted via both catalytic converters, which are connected in series, or via only one of the two catalytic converters. An exhaust system of this kind is relatively expensive since it requires a multi-position actuating element and a multitude of line connections. Furthermore, both catalytic converters, which are connected in series, experience a continuous flow through them of the exhaust gas from the cylinder group that is not switched off. This can lead to a rapid aging. Furthermore, a series connection of both catalytic converters leads to different operating temperatures and heating behaviors. The temperature of the first catalytic converter is very high, while that of the second is relatively low. This also results in slower heating of the second catalytic converter.
German Patent document DE 37 40 238 A1 discloses an exhaust system for an internal combustion engine with two banks of cylinders, in which each of the two exhaust pipes is split into two partial flows and in which a partial flow from the one bank of cylinders is combined with a partial flow from the other, before reaching each catalytic converter. In this way, a largely identical combination of the exhaust gases flowing through the catalytic converters is claimed to be achieved. No provision is made for switching off cylinders.
Furthermore, German Patent document DE 43 32 086 A1 discloses an exhaust system for a two-row internal combustion engine in which an alternative shortening or lengthening of the exhaust pipe upstream of the catalytic converter, depending upon the exhaust temperature, is possible via a shutoff element in each of the two exhaust pipes and lateral connections disposed upstream of the two exhaust pipes.
There is therefore needed an exhaust system for an internal combustion engine, having a group of cylinders that can be switched off, which is simply designed and in which the catalytic converter associated with the cylinder group that can be switched off experiences a flow through it to the extent that it does not cool too severely after the cylinder group is switched off. This is so that it is possible for the catalytic converter to rapidly become effective after the cylinder group is again switched on.
These needs are met according to the present invention by an exhaust system for a multi-cylinder internal combustion engine with a first group of cylinders that cannot be switched off and a second group of cylinders that can be switched off, with exhaust lines associated with each group which start from exhaust manifolds, with at least two catalytic converters in the exhaust system, and with an actuating member in one of the exhaust lines. A first catalytic converter is disposed in the exhaust line associated with the first group of cylinders and a second catalytic converter is disposed in the exhaust line associated with the second group of cylinders. A bypass line is disposed between the first and the second exhaust lines. The actuating member is disposed downstream of the first catalytic converter. The actuating member is a shutoff element which unblocks an opening when the second group of cylinders is switched on and at least partially closes the opening when the second group of cylinders is switched off.
If a catalytic converter is disposed in each of the exhaust lines of the constantly operated cylinder subassembly and the cylinder subassembly that can be switched off, a bypass line runs between these two catalytic converters. The exhaust pipe of the catalytic converter, which is associated with the constantly operated cylinder group, can then be totally or partially shut off or throttled downstream of the catalytic converter. As a result, cooling of the catalytic converter of the cylinder group that can be switched off must be largely prevented after they are switched off. The exhaust system is simply designed since it does not require a large number of pipeline connections or branchings and the actuating element connected downstream can be provided as a simple shutoff element or throttle valve.
Cooling of the catalytic converter associated with the cylinder group that can be switched off can be prevented particularly well and simply if the bypass line begins at the exhaust line of the constantly operated cylinder subassembly, upstream of the catalytic converter associated with this cylinder subassembly. The bypass line feeds into the exhaust line of the cylinder subassembly that can be switched off, upstream of the corresponding catalytic converter. When the shutoff element is completely closed, therefore, the very hot exhaust flow, which has branched off upstream of this catalytic converter, can heat the catalytic converter disposed in the shut off exhaust pipe due to pulsations. When the shutoff element (throttle valve) is partially closed, splitting of the exhaust flow from the constantly operated cylinder subassembly can be achieved, so that both catalytic converters are acted upon by a partial exhaust flow. This largely prevents cooling of the catalytic converter of the cylinder subassembly that is switched off.
Good heating and rapid restarting of the catalytic converter of the cylinder group that is switched off is achieved with a short pipeline length if, in the exhaust flow direction, the bypass line is connected to the exhaust line of the constantly operated cylinder subassembly downstream of the catalytic converter and upstream of the shutoff element. The bypass line feeds into the exhaust line of the cylinder subassembly that can be switched off, upstream of the corresponding catalytic converter. The bypass line can be connected directly to the outlet side of the catalytic converter so that long line paths are prevented and the hot partial flow of exhaust gas is supplied directly to the second catalytic converter.
Inserting a third catalytic converter in this bypass line prevents an unpurified partial exhaust flow from escaping via the bypass line, the outlet side of the first catalytic converter, directly into its outlet side exhaust line when both cylinder groups are operating and therefore the shutoff element is open.
An exhaust system of this type is particularly suited for engines with two rows of cylinders, of which one row of cylinders is constantly operated and the other row of cylinders can be switched off, since in this way, the expense of providing lines can be kept particularly low, and the combining of the exhaust volumes of the individual cylinders is possible in a particularly simple way.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.